The invention relates to a system for remotely detecting a physical magnitude and of the type that operates by reflection, the system comprising an incoherent light source connected by an optical fiber to a sensor having a birefringent sensitive component subjected to the magnitude to be measured and within which light is subject to periodic modulation of its spectrum.
Such sensors are already known (the Applicants' French patent 2 595 820) and are used for remotely measuring physical magnitudes such as temperature, magnetic field, position, displacement, etc. . . . .
With these sensors, the end of the optical fiber is placed at the focus of a collimator lens whose optical axis looks into the sensitive component. When the sensor is of the type that operates in transmission, the sensitive component is preceded by a polarizer and is followed by an analyzer, and a second collimation lens connects it to an optical fiber leading to apparatus for analyzing the spectrum of the light.
When the sensor operates by reflection, the sensitive component is still preceded by a polarizer, but it is followed by a mirror that reflects the received light flux and causes it to pass back through the sensitive component, the polarizer, and the collimator lens to send it back into the optical fiber towards a spectrum analysis apparatus connected to said fiber by a Y-coupler.
Because of the very small size of the core of the optical fiber, the reflecting mirror must be disposed perpendicularly to the optical axis of the system with a very high degree of precision to ensure that a maximum amount of light flux is returned to the optical fiber. For example, if it is desired to obtain a reinsertion loss of the light into the optical fiber, of less than 1 dB, then the angle between the perpendicular to the mirror and the optical axis of the system must be less than three minutes of angle for an optical fiber having a core diameter of about 0.1 mm and for a collimator lens having a focal length of 10 mm.
Such accuracy in mirror positioning is difficult to achieve and to conserve over time.
To avoid this drawback, conventional solutions consist in increasing the diameter of the optical fiber core or in reducing the focal length of the collimator optics. Nevertheless, it is difficult to reduce the focal length below 1 mm, or to use optical fiber having a core diameter that is greater than 0.2 mm. In addition, this results in a collimated light beam with very high divergence and that also gives rise to a loss of light flux that is significant when the collimator lens to mirror distance becomes large relative to the focal length.
Another conventional solution consists in defocusing the optical fiber relative to the collimator lens. This nevertheless gives rise to large loss when the light flux is reinserted into the optical fiber, which loss is a function of the defocusing and may be about 5 dB to 10 dB.
An object of the invention is to provide a solution to this problem which is simple, effective, and reliable, while guaranteeing minimum reinsertion loss.
Another object of the invention is to increase the performance of a sensor of the above-mentioned type.